Cathode ray tube with composite multiple glass fibre face



ay 23, 1967 J. 5. COURTNEY-PRATT 3,321,658

GATHODB RAY TUBE WITH COMPOSITE MULTIPLE GLASS FIBRE FACE Original FiledAug. 26, 1957 3 Sheets-$heet l a 27 i2 3& 9g 14 May 23, 1967 J. 5.COURTNEY-PRATT y fi CATHODE RAY TUBE WITH COMPOSITE MULTIPLE GLASS FIBREFACE Original Filed Aug. 25, 1957 3 Sheets-Sheet 2 BMW:

3 1" MWMkCJWMKFP immeii May 23, 1967 J. 5. COURTNEY-PRATT 3,321,658

CATHODE RAY TUBE WITH COMFQSTTE MULTIPLE GLASS FIBRE FACE Original FiledAug. 26, 1957 5 Sheets-Sheet United States Patent O 3,321,658 CATHODERAY TUBE WITH COMPOSITE MULTIPLE GLASS FIBRE FACE Jeofry StuartCourtney-Pratt, Springfield, N.J., assignor to National ResearchDevelopment Corporation, London, England, a British corporationApplication Dec. 19, 1963, Ser. No. 332,984, now Patent No. 3,141,105,dated July 14, 1964, which is a continuation of application Ser. No.680,308, Aug. 26, 1957. Divided and this application July 2, 1964, Ser.No. 380,006 Claims priority, application Great Britain, Sept. 17, 1956,28,407/56 28 Claims. (Cl. 313-92) This is a division of application Ser.No. 332,984, filed Dec. 19, 1963, now Patent No. 3,141,105. ApplicationSer. No. 332,984 is a continuation of application Ser. No. 680,308,filed Aug. 26, 1957, and now abandoned.

This invention relates to electronic tubes of the type in which an imagecapable of being photographed is projected upon a wall of the tube.

According to the invention there is provided an electronic image-formingtube in which at least part of the envelope of the tube consists of abundle of light guiding fibres bonded together into a gas-tight slabwith the axes of the fibres running transversely from one surface of theslab within the tube to another surface of the slab outside the tube,the slab being bonded to the remainder of the envelope in gas-tightfashion so as to form a structural wall of the tube.

According to the invention there is further provided an electronic tubecomprising a source of electrons, means for accelerating the electronsso that they bombard a wall of the tube, means for controlling theelectrons so that they provide an image-forming beam, the wall of thetube bombarded by the electrons being in the form of a bundle of lightguiding rods bonded together into a gas-tight slab with the fibresoriented in a regular array and with their longitudinal axes extendingfrom the interior surface of the tube to the exterior surface of thetube, the surface of the slab within the tube being coated with asubstance of a type which, when bombarded with electrons, emitsradiation to which a photographic plate is sensitive.

According to the invention, there is yet further provided an electronictube assembly comprising a first photosensitive cathode layer depositedupon a first wall of a first evacuated envelope, a first phosphor layerdeposited upon a second Wall of the first evacuated envelope, such wallbeing in the form of a bundle of light guiding fibres bonded togetherinto a gas-tight slab, with the fibres oriented in a regular array withtheir longitudinal axes extending from one major surface of the slab tothe other, means for accelerating the electrons emitted by the firstcathode layer so that they bombard the first phosphor layer, means forfocusing the said electrons into an imageforming beam, at secondphososensitive cathode layer deposited upon a third wall of a secondevacuated envelope, the third wall being in the form of a bundle oflight guiding fibres bonded together into a gas-tight slab, with thefibres oriented in a regular array with their longitudinal axesextending from one major surface of the slab to the other, the secondand third walls being juxtaposed so that light given off by the firstphosphor layer passes along the light guiding fibres of the second andthird walls in suc cession and activates the second photosensitivecathode layer, a second phosphor layer deposisted upon a fourth Wall ofthe second envelope, the fourth wall being in the form of a bundle oflight guiding fibres bonded together into a gas-tight slab, with thefibres oriented in a regular array With their longitudinal axesextending from one major surface of the slab to the other, means foraccelerating electrons emitted by the second photosensitive cathodelayer so that they bombard the second phosphor layer, and means forfocusing the said electrons into an image-forming beam.

The invention will be more readily understood from the followingdetailed description illustrated by the accompanying drawings in which:

FIG. 1 is a diagram illustrating certain disadvantages encountered intaking photographs of the display of an electron tube of conventionaltype.

FIG. 2 is a section of part of a tube according to the invention.

FIG. 3 shows, diagrammatically, an electron tube assembly incorporatingthe invention.

FIG. 4 is a partial end view of a bundle of light guiding fibres for usein the invention.

FIG. 5 illustrates by means of a perspective drawing a method of makinganother type of bundle of light guides for use in the invention.

FIG. 6 is a partial end view of a bundle of light guides made by themethod illustrated in FIG. 5.

FIG. 7 illustrates by means of a perspective drawing another method ofmaking a bundle of light guides which is a modification of those shownin FIGS. 6 and 7.

FIG. 8 is a partial end view of a bundle of light guides made by themethod illustrated in FIG. 7.

FIG. 9 illustrates by means of a perspective drawing a method, similarto that illustrated by FIG. 7, of making a bundle of tapered lightguides.

FIG. 10 is a side elevation of a bundle of light guides made by themethod illustrated in FIG. 9.

FIG. 11 illustrates by means of a perspective sketch a further method ofmaking a bundle of light guides for use in the invention, and,

FIG. 12 is an end elevation of part of a bundle of light guides made bythe method illustrated in FIG. 11.

For explaining the nature of the invention, it is convenient to considera cathode ray tube with a fluorescent screen such as is used in cathoderay oscillographs or television receivers, though, as will be explainedbelow, the invention is not confined to such tubes.

In a cathode ray tube, a beam of electrons strikes a fluorescent orphosphorescent screen which normally takes the form of a coating appliedto the inner surface of a glass wall of the tube and beam deflectingmeans and beam intensity controlling means are provided so that an imageis produced on the screen in response to signals applied to these means.Considering a single illuminated picture element of the image, some ofthe light is scattered, some is absorbed, and some pases through theglass Wall and is emitted outwardly from the wall of the tube indivergent directions. The distribution of the latter is such that theintensity is greatest in directions normal to the tube wall anddiminishes as the angle to this normal increases and it is generallyconsidered to follow a Lambert distribution though there are indicationsthat the reduction of intensity with increasing angles to the normal issomewhat more rapid. Nevertheless an observer in the most favorableposition sees only a small fraction of the light emerging from the tubewall and in a typicalcase a lens placed at a suitable position forphotographing the image received only one percent of the total lightemerging from the tube wall.

It is possible by the use of special lenses to improve on thisperformance somewhat but even so the major part of the light emittedfrom the tube is lost.

This is illustrated in FIG. 1 which shows a cross section of the endwall of a tube of the type in question. The lines 1, 2, bearing arrows,to the left of the figure, represent a focused electron beam impingingupon an area ab of the fluorescent coating 3 on the glass-end wall 4 ofthe tube to form the normal scanning spot. When viewed by the human eyeor an equivalent manmade lens system, such as the camera lens 5indicated diagrammatically at the right hand side of the figure, only asmall part of the emergent light, namely that which is represented bythe arrow-marked lines 6 joining a-b to the lens 5, strikes thephotographic plate indicated diagrammatically at 9. The divergent arrows7 and 8 indicate the light missed by the lens 9 and wasted. This is notgreatly affected by the thickness of the glass wall of the tube. Thelight is transmitted in all directions from the spot onthe fluorescentcoating and any rays not suffering total internal reflection in theglass receive the same additional dispersal due to refraction at theglass-air interface, whether the glass is thick or thin.

It has been proposed to overcome this difficulty by placing a sensitizedplate within the evacuated area of the tube so that the electronsproduce a latent image directly in the emulsion, but this involves thecomplication of a continuously evacuated tube with a detachable wall andother well known disadvantages of such tubes.

Another proposed solution of the problem is to make a wall of the tubeof metal so thin that the electrons of the cathode beam can pass throughit and expose a sensitized plate placed against the wall outside thetube. The structural weakness of such a thin wall is such that it willnot support the vacuum within the tube unless there is is a vacuum onthe outside of the wall as well as the inside; and the provision of thisexternal vacuum involves a continuously evacuated ante chamber which islittle less troublesome than a continuously evacuated tube with thesensitized plate placed inside it.

Yet another proposed solution is to provide a tube with a substantiallyflat image-bearing wall and to place a sensitized plate directly incontact with it. The disadvantage of this proposal is that, althoughmost of the emergent light is captured, the thickness necessary toenable a flat wall to withstand the vacuum within the tube is such thatthe light spreads out, within the glass, in a cone of substantialincluded angle so that a light spot on the phosphor within the tube isspread out over a considerably increased area by the time it reaches thesensitized plate and resolution is much impaired. In a typical case, aresolution of the order of 2 to 3 lines per centimetre was the best thatcould be obtained. It is no solution of this difficulty to increase thescreen size and attempt to restore the definition by photographicreduction of the exposed plate since structural strength requires astill thicker wall which cancels out the advantage of increased imagesize. These difficulties can be illustrated by the aid of FIGURE 1,wherein it is seen that the light spot a-b has become expanded to anarea indicated by the lines c-d causing overlapping with a light spot ina position removed from the first by several times the diameter of thespot. The dotted lines 10 and 11 show such an adjacent beam and spotposition which is dispersed to the area indicated at 12- the top edge ofwhich comes approximately in the middle of the area c-d. The sharpwhite-black-white transitions between the a-b position and the dottedline 10, 11 position on the fluorescent screen are completely obscuredby the patch of light between 0 and 1.

These difficulties are overcome, according to the invention by the useas part of the external envelope of the tube, of a bundle oflight-guiding fibres bonded together into a gas-tight slab with thefibres running transversely from one surface of the slab to the other.

The image-forming rays are directed to the interior surface of the slabwhich in the case of a cathode ray tube will be coated with a layer offluorescent or phosphorescent material.

The principle of light guides is so Well known as to need only briefdescription. A transparent elongated smooth-surfaced body of higherrefractive index than its surroundings will transmit light applied toone end so that it emerges with little loss from the other end, due tointernal reflection from its surfaces of light rays divergent from thelongitudinal axis of the body. Where a bundle of light guides is used,an image cast on one end face is reproduced with little loss of lightintensity upon the opposite end face even when the individual lightguides are of substantial length. The output end of each individuallight guide is evenly illuminated to a degree depending on the averageintensity of the light falling on its other end upon which the image iscast so that the resolution of the image after transmission along thebundle depends upon the number and cross-sectional dimensions of theindividual guides. For most practical purposes, the individual lightguides should take the form of fibres which, where extremely highresolution is required, may be very fine, for instance of the order of0.001 inch in diameter or even finer, though requirements for fibresfiner than about 0.004 inch in diameter will seldom arise in practice.

It is necessary that the same relative positions shall be maintained bythe fibres at both ends of the bundle as any substantial transpositionof the fibres would result in a scrambled image appearing at the viewingend. An occasional rod out of place here and there is of littleconsequence however.

The fibres should be bonded together with a substance capable of fillingthe interstices between the fibres and uniting with the surfaces of thefibres to provide a gastight slab; and this bonding material should havea relatively low refractive index in relation to that of the material ofwhich the fibres are made as this is a condition necessary for theinternal reflection upon which light guides depend for their action. Thebonding material should also be of such a nature that it does not giverise to gassing troubles when exposed to the conditions within theevacuated tube. It is not essential for the bonding material meetingthose requirements to extend through the full thickness of the slab solong as it extends to a suflicient depth from the interior surface ofthe slab to ensure gastightness and freedom from outgassing since theremaining thickness of the slab may be bonded with a material not havingthose properties so long as it provides the required structural strengthand has a refractive index lower than that of the fibres. It isfurthermore not essential that the light guiding fibres should be bondedtogether over their full length as long as the bonded region issufficient to produce the required strength and gas-tightness. Ifmaterial to meet the above requirements is not available, the bundleshould be protected from the conditions within the tube by a thin layerof transparent material applied to the interior surface of the bundle.This will cause some scattering of the light within the thickness of thecoating but the loss of definition will not be serious if the coating isthin. If difficulty is experienced in obtaining a bonding materialhaving a suitable refractive index and at the same time possessing theother qualities required of it, the difficulty may be overcome bycoating fibres with a thin layer of a reflecting metal, such as silveror aluminum, before assembling the bundle, or alternatively with a thinlayer of material of low refractive index. The latter is in factsuperior to the former as metals, even with a polished surface, absorb asubstantial amount of incident light. The bundle is sealed into andforms part of the wall of the tube and it may be of any convenientthickness which may be required to withstand the vacuum. Both surfaceswill generally be ground flat and parallel to one another, but this isnot fundamental,

to the invention in all its applications. For instance, it may beconvenient for certain applications to have the outside surface of thebundle in the form of part of the surface of a cylinder. The importantthing is that the wall can be made in shapes which would not, in theabsence of the invention, be strong enough to withstand the vacuumunless they were of such a thickness as to impair definition to anunacceptable extent. A wall according to the invention can be of anyconvenient thickness without affecting image definition to anyappreciable extent.

Where it is intended to photograph the image, the outer surface of thebundle is generally made flat and a sensitized plate is placed incontact with it. By this means, the light which would be lost by using alens system is saved and there is not the degradation of definition dueto diffusion in the thick Wall which is necessary to withstand thevacuum when the wall is of homogeneous material and flat so that thesensitized plate can be placed in contact with it to avoid the need foran image-forming lens system.

FIGURE 2 shows a cross section of part of the fibre bundle 12 and partof another wall 13 of a cathode ray tube according to the invention,used for photographing the tube display on a flat photographic plate 14.The electron beam 15 shown on the left, lights up a spot on the phosphorscreen which bridges at least part of tour fibers 16, 17, 18 and 19, asseen in section, and each fibre is shown with internal reflectionsproceeding along it, at different angles in the four fibres though infact, of course, each fibre has reflections at all angles above thecritical angle. The diameters of the fibres have been greatlyexaggerated so that the paths of internally reflected light rays can beshown. The light path which exposes the sensitized plate may be blurredslightly at the edges by reason of the edge of the light spot on thephosphor screen over-lapping only part of a fibre. This will producereduced illumination evenly distributed over the whole of the other endof the fibre, but as in practice, the spot on the phosphor screen seldomhas a sharp margin, the loss of definition is small so long as thecross-sectional areas of the individual fibres are small compared withthe image. The plate 14 is preferably placed with the emulsion 21towards the tube. In FIG. 2, the thickness of the emulsion 21 has beenexaggerated.

The invention may with special advantage be applied to image convertertubes of the type having a photosensitive cathode, an electronicfocussing system, and a fluorescent screen, accelerating potentialsbeing provided for producing on the screen an image corresponding to theimage cast upon the photo-sensitive cathode, the intensity of the imagebeing increased by a factor which may be between 10 and 100. The amountof light leaving the screen which can be collected by a lens is howeverrarely more than about 1 percent of the total light emitted by thescreen so that there is an overall loss of intensity or at any event nogain. Similarly, little or nothing is gained by arranging several suchtubes in cascade where a lens system is interposed between the screen ofone tube and the cathode of the next. By means of the invention however,the screen of such a tube can be made fiat and for photographing theimage thereon, a photographic plate may be placed against its outsidesurface thus avoiding the loss of intensity associated with the use of alens system without the sacrifice of resolution due to dilfusion in ahomogeneous tube wall of comparable thickness.

A further important advantage of the invention as applied to imageconverter tubes is that it enables two or more of them to be used incascade with advantage. This may be done by making the screen wall ofone tube and the cathode wall of the next tube in the cascade, ofbundles or fibres the interior face of the former carrying a fluorescentlayer and interior face of the latter a coating of photosensitiveelectron emitting material. The

outer surfaces of both these bundles of fibres are flat and are placedin contact with one another and it is not essential to register themfibre to fibre as the staggering of the fibres of the two screens causesonly a slight loss of resolution if the fibres are thin. By this meansthe full gain of one tube is effectively passed on to the next tube inthe cascade, only a very small loss being incurred due to absorption inthe material of the fibres and due to the escape of rays striking thewalls of the individual light guiding fibres at angles of incidence lessthan the critical angle for the particular combination of materials usedfor the fibres and the bonding material respectively.

If the amplified image is to be photographed, the screen of the lasttube should be on a wall consisting of a bundle of fibres to enabledirect photographing to be acheiev'ed without the loss of light involvedin the use of a lens system. It will be clear that this latter loss issuffered if the final display is viewed by the eye, since the eye is alens system and catches only a small part of the emergent light. Themaximum efliciency can therefore only be obtained by photographing thefinal display by placing the photographic emulsion directly up againstthe fibre bundle end wall of the final tube.

FIG. 3 shows an arrangement of cascaded image convertor tubesschematically. Three tubes 22, 23 and 24 are shown, the incident imagerays 25 being applied to the left hand wall of tube 22 which carries aphotosensitive cathode coating 26. This wall need not necessarily bemade of a bundle of light guiding fibres but it is necessary that thescreen wall 27 of tube 22, the cathode and screen walls 28 and 29 oftube 23, and the cathode and screen walls 30 and 31 of tube 24, shouldbe so constructed to obtain the full benefit of the invention.

It may be seen from FIG. 3 that this multi-tube cascade can beconstructed as a single tube with two intermediate partitions made ofbundles of light guiding fibres coated on one side with fluorescentmaterial and on the other side with photosensitive electron-emittingmaterial.

These partitions can, substantially without loss of light intensity andresolution, be made of any thickness necessary to withstand the fullinter-electrode potential which may appear across the screen of one tubeand the cathode of the next if a common supply source provides thispotential for all three stages.

Any other form of partition wall in such a tube would have to be made ofphysically thin material to preserve definition and this adds to thedifliculty of manufacture.

Any convenient number of cascade stages can be made as a unit structurein this way.

Such a cascade arrangement may precede a television camera tube toenable it to operate in conditions of low illumination, for instance bystar light.

Where several tubes are cascaded in the manner above described, thecumulative loss of definition due to staggering of the light guides asbetween the phosphor coated Wall of one tube and the cathode coated wallof the next may be unacceptable. This loss may be minimized if, inmanufacture, the bundles of light guides are made with the spacing andarrangement of the fibres accurate and uniform as between one bundle andanother, preferably with index marks to secure uniform orientation.Adjacent bundles in an assembly of cascaded tubes can be turned till theindex marks coincide, and then aligned in two dimensions by micrometeradjusting screws until optimum definition is obtained. Some of themethods hereinafter described of making bundles of light guiding fibres,permit the making of bundles of substantial thickness lengthwise of thefibres. A thick bundle can be sliced into a number of almost identicalslabs from which can be made a batch of matched tubes for use in acascade arrangement. The matched tubes could then readily have theiradjacent walls adjusted for alignment of the fibres even though thearrangement and spacing over the slabs was irregular.

The invention finds uses in other electronoptical apparatus, such aselectron diffraction cameras, electron microscopes and the like. Acontinuously evacuated demountable tube is still necessary, but if theend wall of the tube is made according to the invention, a photographicplate can be placed against it outside the evacuated space and can bechanged or traversed over the outside surface of the wall to take asuccession of photographs of a specimen within the tube which may beundergoing changes during the period of observation. To achieve thisobject without the use of the invention necessitates elaborate remotelycontrolled plate or film Changing devices operating within the evacuatedzone.

Instead of parallel light guiding fibres, tapered fibres may be used.These tapered fibres may be made in a tapered or a parallel format.

In the case of the tapered format, the slab of light guides is of largerarea at the large ends of the fibres as compared with the small ends,the relative spacing of the fibre-ends being substantially the same atboth faces of the slab.

A slab of this type incorporated in a tube with the large ends of thefi-bres inside the tube, can be used for photographing, at reduced size,a display on a phosphor on the surface of the slab inside the tube. Forsubstantial reductions of image size, the brightness in terms ofquantity of light per unit area will be substantially the same at bothfaces of the slabs and there is little or no gain of specific brightnessdue to the reduction in size of the image. The reason for this isexplained below. Nevertheless there is a substantial gain in the lightavailable for exposing a photographic plate as compared with aconventional lens reduction system, by virtue of the plate being indirect contact with the small end of the tapered slab, since the lenssystem working with any substantial reduction of image size, onlycatches a fraction of one percent of the light provided by the displayon the phosphor.

Where a tapered slab is used with the small ends of the fibres insidethe tube, the total quantity of light per rod is substantially the sameat both ends of the guide so that bright large-screen display isobtained at the large end of the slab with only moderate brilliance atthe phosphor as compared with that required with conventional largescreen systems using an optical enlarging system such as a foldedSchmidt lens system where all but a small percentage of the light givenout by the phosphor is wasted. This arrangement is of great value forproviding large television or radar displays.

A tapered light guide in a parallel format provides at the thin ends ofthe fibre rods, an image built up of widely spaced dots resembling ahalf-tone picture. This may be used for high speed cinematography of aquickly changing display on the phosphor of the tube. The procedure isto displace the photographic plate by a small amount for each exposureso that the dots fall in different but interlaced positions. As theplate can be displaced in two directions at right angles the number ofseparate exposures is proportional to the square of the interdotspacing. A similar effect can be achieved by using parallel guides and asuitable mask exposing only part of the outer end face of each guidefibre.

The final result is a Series of interlaced half tone images which can beunscram-bled in a number of ways well known in the high-speedcinematography art. Unscrambling can proceed at leisure so that simpleapparatus may be used. A screen with holes in the same formation as thesmall ends of the fibres in the slab may be readily produced byphotographing the slab with even illumination of the phosphor. Such ascreen can be placed over the scrambled negative and successive printstaken with the screen exposing different sets of dots for each print.

Cit

With tapered light guides, there is a difference in performance asbetween convergent and divergent guides, in the direction of lighttransmission. This may be explained as follows. Take the case of aparallel guide fibre: with the optimum ratio of refractive indices asbetween the fibre and its surroundings it can be relied upon that alight ray incident upon the input end of the fibre, at almost grazingincidence, will be refracted on entering the guide to an angle morenearly approaching parallelity with the axis of the guide and that sucha ray will have an angle of incidence greater than the critical angle,to the surfaces of the guide, this angle remaining constant despitemultiple reflections in passage of the ray along the guide. In the caseof a convergent tapered guide, however, the angle of incidence of rayson passage along the guide is progressively diminishing and certain rayswhich would have passed along a parallel guide are lost in the case of aconvergent tapered guide when the changing angle of incidence becomesless than the critical angle. This loss becomes progressively moresevere, the greater the reduction in area as between one end of theguide and the other.

Where a 10:1 reduction of linear dimensions is used (100:1 in area), forinstance, there is a loss of all but about 1 percent of the ingoinglight which accounts for the brightness per unit area beingsubstantially the same at both ends of a convergent light guide slab ina tapered format, as stated above. Where a tapered format is notrequired, a similar effect to a parallel format of tapered light guidescan be obtained by using parallel guides and masking all but, forexample, of the area of the output ends of the individual guides, theloss of light being about the same at substantial size reduction ratiosas that suffered by tapered light guides in a parallel format.

With divergent tapered light guides however, the angles of incidence ofrays increase on successive reflections on passage down the guide andrays which would just escape reflection in a parallel guide will bereflected and retained in a divergent guide. This explains why there isno loss of specific brilliance in the case of divergent light guides.

Various methods of making a bundle of light guiding fibres will now bedescribed.

For extremely high definition where the fibres must be of smalldiameter, for instance a few thousandths of an inch or less, it ispreferred to use, as the fibres, short lengths of glass filament drawndown to the required size and bonded together, for at least part of thethickness of the slab, with a glass of lower melting point and a lowerrefractive index than that of the fibres. Glasses of the type commonlycalled glass solders are suitable for the purpose. FIGURE 4 shows aportion of a major surface of such a slab, on an enlarged scale.

To secure the maximum possible transmission of light along the fibres,the refractive indices of the fibres and the surrounding material incontact with their surfaces of the slab must have the relationship n 1+nwhere 11 is the refractive index of the fibres and n is the re fractiveindex of the surrounding material.

Table I below shows some typical examples:

TA l3 LE I Refractive index of material surrounding the fibres (11Refractive index of fibres (71 (equal to or greater than) Table II belowshows the consequences of departing from the required relationship in acase where 11 is fixed at 1.5 and n is yaried:

1 Zero internal reflection only direct rays pass.

In the above tables emphasis is placed on n because it is difficult toobtain vitreous materials with a refractive index below about 1.4, andthis determines the range of glasses from which the glass for thefi-bres may be chosen.

An alternative to high melting point fibres embedded in low meltingpoint glass solder is illustrated in FIG- URES 5 and 6.

A glass of relatively high melting point and low refractive index ischosen for the material surrounding the fibres. Strips of this glass inthin sheet form, preferably of the order of 0.001 inch thick, arecorrugated With the corrugations running transversely of the length ofthe strip. The corrugations may be imparted to the strips by passingthem when hot between cogged rollers or pressing them between matchingribbed plates. The profile of the corrugations is not critical providedthat the bends are not so abrupt as to weaken the strips. Anapproximately sinusoidal profile is suitable. These corrugated strips,shown greatly enlarged at 32 in FIGURE 5, are interleaved withuncorrugated strips of the same material, shown at 33 in FIGURE 5, and ahoneycomb structure is built up of the size required for the slab. Thespaces between the corrugations and the flat interleaving strips are nowfilled in with a glass of low melting point and high refractive indexwhich will Wetthe glass of the honeycomb, and this glass forms fibres.The advantage of this arrangement over that previously described is thatglasses of high refractive index and low melting point are in generaleasier to handle than glasses of high refractive index and high meltingpoint.

FIGURE 6 shows an end view of a section of a slab made by the methodillustrated in FIGURE 5 to a somewhat enlarged scale. It is importantthat the height of the corrugations should be accurately uniform toavoid gaps between their crests and the interleaving plates, which wouldprovide interconnecting webs between adjacent fibres. Some slightleakage between adjacent fibres is inevitable if the filling-in glass isto seal the slab completely, but the leakage of light between adjacentfibres will not execeed a few percent so long as the thickness of anysuch web interconnecting the fibres is less than about one fortieth ofthe thickness of the fibres.

Where the highest definition is not required, the light guides may bemade in the form of ridges raised from the surface of a sheet ofsuitable material. Ridged sheets of this type are shown in FIGURE 7where six sheets 34, 35, 36, 37, 38 and 39 are shown as part of anassembly, in exploded form, which is destined to form part of a bundleof light guiding fibres. The ridged faces such as 40, 41 of the sheetsare so shaped that they interlock when brought together and adjacentflat faces such as 42, 43 also come together. These ridged and adjacentfaces are bonded together with a material having a lower refractiveindex than that of the sheets so as to form a solid block an end view ofa part of which is shown in FIGURE 8. Sheets such as 3439 may be made inglass by a moulding or casting process and assembled in clamps and thinspacers may be placed between adjacent sheets along two opposite edgesof the sheets to leave spaces between adjacent faces into which thebonding material can be run. Where these spacers run across the surfaceof the slab the part of the slab which they occupy is preferably groundaway when bonding has been completed.

It has previously been indicated that a bundle of tapered light guidescan be used with advantage for image reduction or enlargement when theguides are made up in a tapered format.

FIGURE 9 shows a ribbed sheet 44 made by a process similar to that usedfor the sheets of FIGURE 7 but in this case the ribs are tapered inwidth, spacing and height from one end to the other. An assembly of suchsheets, the actual thickness of the individual sheets being somewhatenlarged to simplify the drawing, is shown in side elevation in FIGURE10.

Where single fine fibres are used as the light guides, there may bedifiicuty in assembling them into a block so that they take up a regularformation. One way of ensuring a regular formation is to weave thefibres into a fabric with transverse connective strands considerablyfiner than the fibres themselves. Several strips of such fabric are thenstacked on top of one another to form the slab, and if the edges aretrimmed so that the angle of the fibres to the edge (preferably isuniform, the parallel alignment of the fibres in adjacent layers of thestack is readily ensured by dressing the edges of the sheets against aplane surface.

In making such a fabric, a simple loom may be used and the connectingstrands are preferably used as the warp since they need not be closelyspaced and thus closely pitched and slender reeds are not required forthe heddles. With the fibres used as the weft, little difiiculty isexperienced in ensuring that all the bending is confined to theconnecting strands, since they are positively bent around the thickerfibres of the weft when the heddles are moved to change the shed. It ispreferable to introduce the fibres into the shed of the warp by means ofa rapier rather than a shuttle as sharp bending and the formation of aselvage at the edges is undesirable. Such a rapier is thrust through theshed and grasps the end of a reel of fibre material by means of a clampon its end. The rapier is then withdrawn and the end of the fibrematerial is drawn through the shed, being severed from the reel by achopping mechanism located on the other side of the Warp, that is to saythe reel side, in such a way that a free end projects from the reel tobe grasped by the rapier on its next excursion through the shed.Mechanisms of this type are well known in the textile industry.

The preferred weave is a twill weave in which each warp strand passesalternately over two and under two weft strands, the crossing pointadvancing by one weft strand as between one warp strand and the adjacentone in a particular direction across the warp. The fabric thus woven issuperior to a plain weave as the latter will stretch warp-wise when allthe bending is done by the warp. With the twill weave however, as shownin FIG- URE 11, for every pair of warp strands such as 45, 46 whichcross over between two adjacent weft strands such as 47, 48, there is anadjacent pair of warp strands such as 49, 50 binding those two weftstrands together.

A part of a stack of fabric sheets in shown in end view, greatlyenlarged, in FIGURE 12 and it can be seen that if the stack iscompressed vertically the weft fibres of one layer will sink into thespaces between adjacent fibres in the adjoining layer depresisng strandsof the warp which bridge pairs of adjacent fibres so as to draw thefabric tightly together warp-wise. The warp ends of the sheets must ofcourse be appropriately sealed to prevent fraying.

Another method of providing a web fibre of regular formation is to winda continuous length of fibre material on to a cylinder of large diameterwith regular closely spaced turns in a single layer. The completed layeris then impregnated with the chosen bonding material to form websjoining adjacent turns together and the coil is then cut into slices,the cuts running parallel to the axis of the cylinder. The slices whenstacked and ll clamped together with the cut edges parallel will thennest accurately together with the fibres in regular formation. Furtherimpregnation with the chosen bonding material turns the stack into asolid slab.

In a modification of this method, a multilayer winding with the turns ofadjacent layers accurately and regularly aligned (which necessitates thewinding of all the layers in the same axial direction), is built up to asubstantial radial thickness. The coil is then impregnated as a wholewhilst still supported by the winding drum. Individual slabs can then becut from the coil by pairs of cuts in planes parallel to one another,parallel to a radius between the planes and parallel to the axis of thedrum.

In cases where difficulty is experienced in using materials for the slabwhich will withstand the temperatures necessary for out-gassing the tubeor where for any reason, such as expense, it is deliberately decided touse materials which would give off gases which would contaminate theelectrodes of the tube, the slab may be isolated from the interior ofthe tube by a thin layer of glass bonded to the surface of the slab. Theprice of using this layer of glass is some loss of definition sincethere is some divergence of the light rays in passage through thethickness of the glass. This loss of definition will be small however solong as the glass is thin in relation to the diameter of the fibres andthe glass will inevitably be fragile. In making a tube of this type itwill therefore be necessary to evacuate the tube and heat it forout-gassing, with the thin glass sheet united to the walls of the tubeas a flat membrane. To avoid rupture of the membrane on evacuation ofthe envelope, the exterior of the tube, at least at the membrane end,must also be enclosed in an evacuated space to equalise the pressure onboth sides of the membrane. After heat treatment is completed, the slab,coated with a suitable cement, is then introduced through a vacuum lockinto the enclosure outside the tube and brought into contact with theouter surface of the membrane which may be slightly bowed outwards bymeans of a slightly lower pressure in the enclosure outside the tubethan that within the envelope, so that contact is secured between themembrane and the slab over the whole of their adjacent surfaces when theslab is pressed against the membrane by a remotely controlled handlingmechanism within the evacuated enclosure surrounding the tube. When thecement has set and united the slab to the membrane, the vacuum can bereleased from the said enclosure and the membrane will be supportedagainst collapse by the adherent slab.

When this form of construction is used, the choice of materials for theslab is greatly widened and both fibres and bonding material may be ofmaterials other than glass. As an instance, fibres of an acrylic resin,may be bonded together with an epoxy resin adhesive. This ribbed sheettype of fibre assembly illustrated in FIG- URES 7, 8, 9 and 10 is welladapted for use with such materials as the ribbed sheets can be producedby well known moulding techniques whereas the moulding of such sheets inglass necessitates the use of special metals for the moulds to ensureready parting of the moulding from the dies.

With this method of tube assembly, the membrane completes and ensuresthe gas-tight sealing of the slab so that, in choosing the bondingmaterial for filling the interstices between the fibres, preference canbe given to a material with a suitable refractive index even though itssealing properties would be inadequate in the absence of the membrane.With this method of tube assembly, it is also possible to clamp thelight guiding fibres or sheets of fibres together, relying solely on themembrane to provide the gas-tight sealing, the friction of the clampingand the adhesion of the guides to the membrane providing the strengthrequired of the slab. References in this specification to a gas-tightslab shall be deemed to include a slab where gas-tightness is securedcompleted or ensured by the use of a membrane as above described and insuch a slab the membrane is to be regarded as part of the slab.Furthermore, in the case of slabs of this type, references to the lightguiding fibres as extending from one surface of the slab to the otherare to be regarded as referring to the orientation of the axes of thefibres and not as implying that the rods penetrate the membrane.

Mention has been made previously of coating the light guiding fibreswith a material of low refractive index to enable the bonding materialto be chosen without regard to its refractive index. When the materialsof which the slab is made are not required to be capable of withstandingthe temperatures used in heat treating the tube, plastic substances maybe used for coating the fibres. Polychlortrifiuorethylene, having arefractive index in the region of 1.43 and which may be caused toadhere, for instance, to glass, is a suitable substance for the purpose.This material may be applied in liquid form by spraying or dipping andcured at a temperature of about 280 C. to produce a continuous adherentfilm.

To provide a slab resistant to higher temperatures than this, a vitreoussubstance must be used for coating the fibres. In one process, a layerof vitreous quartz of refractive index about 1.48 is applied in the formof a silicone or chlorsilane to the fibres whose temperature is held atabout 400 C.

To be effective in securing total internal reflection within fibrescoated in this way, the thickness of the coating should preferably beseveral half-wave-lengths of the light requiring to be transmitted,though a thickness of only one half-wave-length will suffice in certainapplications.

In the case of tapered guides used in a parallel format, i.e. with theiraxes parallel, arrangements must be made to space the narrow ends apartin the correct formation, during impregnation with the bonding material.One method of achieving this, where individual tapered fibres are used,is to weave them together in the manner illustrated in FIGURES 11 and 12but with the warp woven only along the thin ends of the fibres so thatthe thick ends are able to make contact with one another.

On assembly of a stack of sheets woven in this way, the sheets must bespaced apart at the thin ends of the fibres by spacers of material oflow refractive index, if the spacers make contact with the rods. It mayhowever be arranged that the warp strands separate the fibres fromcontact with the spacers in which case the refractive index of thelatter in unimportant.

References in this specification to light guiding fibres shall be deemedto include fibres which perform the analogous function when subjected toelectromagnetic vibrations having properties equivalent to those ofvisible light notwithstanding the fact that such vibrations are outsidethe range of vibrations to which the human eye is sensitive.

What I claim is:

1. In an electronic image-forming tube having an envelope, a honeycombstructure forming at least part of said envelope and comprising lightguiding fibres bonded together into a gas-tight slab, the axes of saidfibres extending along transverse lines from an interior surface of saidslab to an exterior surface thereof, said fibres including corrugatedglass sheets having the corrugations thereof extending parallel to saidtransverse lines, fiat glass sheets interleaved between said corrugatedglass sheets and cooperating to form internal volumes between adjacentcorrugated and flat sheets, and glass fillings in said internal volumesto build up fibre cores in the form of said honeycomb structure in whichsaid fibres are separated from each other by said glass sheets, saidcorrugated glass sheets, said fiat glass sheets and said glass fillingsof said honeycomb structure being bonded to each other in gas-tightfashion, said honeycomb structure being bonded to said envelope ingas-tight fashion so as to form a structural wall of said tube.

2. An electronic image-forming tube according to claim 13 1 in whichsaid glass sheets are substantially .001 inch in thickness and formedfrom glass having a relatively low melting point and low refractiveindex.

3. An electronic image-forming tube according to claim 1 in which thecrests of said corrugations are uniform in height such that all of saidcrests contact the interleaved fiat sheets.

4. In an electronic image-forming tube having an envelope, a honeycombstructure forming at least part of said envelope and comprising lightguiding fibres bonded together into a gas-tight slab, the axes of saidfibres extending along transverse lines from an interior surface of saidslab to an exterior surface thereof, said fibres including corrugatedglass sheets having the corrugations thereof extending parallel to saidtransverse lines, fiat glass sheets interleaved between said corrugatedglass sheets and cooperating to form internal volumes between adjacentcorrugated and fiat sheets, and glass having a lower refractive indexthan the glass of said sheets filling in said internal volumes to buildup fibre cores in the form of said honeycomb structure in which saidfibres are separated from each other by said glass sheets, saidcorrugated glass sheets, said flat glass sheets and said glass fillingsof said honeycomb structure being bonded to each other in gas-tightfashion, said honeycomb structure being bonded to said envelope ingas-tight fashion so as to form a structural wall of said tube.

5. A honeycomb structure of light-conducting fibres comprising plurallight guiding fibres bonded together into a gas-tight slab having firstand second surfaces on opposite sides thereof, the axes of said fibresextending along transverse lines from said first surface to said secondsurface, said fibres including corrugated glass sheets having thecorrugations thereof substantially parallel to each other and extendingparallel to said transverse lines, flat glass sheets interleaved betweensaid corrugated glass sheets and cooperating to form internal volumesbetween adjacent corrugated and flat sheets, and glass fillings having alower refractive index than the glass of said sheets in said internalvolumes to build up fibre cores in the form of said honeycomb structurein which said fibres are separated from each other by said corrugatedand said flat glass sheets.

6. In an electronic image-forming tube having an envelope, a honeycombstructure forming at least part of said envelope and comprising lightguiding fibres bonded together into a gas-tight slab, the axes of saidfibres extending along transverse lines from an interior surface of saidslab to an exterior surface thereof, said fibres including glass sheetseach having a ridged face and a relatively flat face, said sheets beingarranged to build up said fibres in the form of said honeycomb structurewith adjacent ridged faces joined together in interlocking relation andwith adjacent flat faces joined together in abutting relation, saidhoneycomb structure being bonded to said envelope in gas-tight fashionso as to form a structural wall of said tube.

7. An electronic image-forming tube according to claim 6 in which saidridged faces and flat faces are bonded together by a material having alower refractive index than said sheets of glass.

8. An electronic image-forming tube according to claim 6 in which saidridges are of triangular cross-section.

9. An electronic image-forming tube according to claim 6 in which saidridges are tapered in width, spacing and height from one end of saidsheet to the other.

10. A block structure of light-conducting fibres adapted to form part ofthe envelope of an electronic image-forming tube comprising plural lightguiding fibres bonded together into a gas-tight slab, the axes of saidfibres exending along transverse lines from an interior surface of saidslab to an exterior surface thereof, said fibres including ribbed glasssheets having the ribs thereof raised from the surface of said sheets ofglass, each of said sheets having a ribbed face and a relatively flatface, said sheets arranged to build up said fibres in the form of saidblock structure such that the ribbed faces of adjacent pairs of sheetsare superimposed with the respective ribs of one sheet interlocking theribs of an adjacent sheet and the flat faces of adjacent pairs of sheetsabutting each other, said ribs having progressively decreasing crosssections beginning at a narrow free end of said ribs and terminating ata wide free end thereof in the form of a tapered configuration, saidblock structure being bonded to said envelope in gas-tight fashion withsaid narrow ends of said ribs disposed inside said envelope to form astructural wall of the tube, the quantity of light at the narrow end ofeach of said light guiding fibres being substantially the same as thequantity of light at the wide ends of the fibres.

11. A block structure of light-conducting fibre according to claim 10 inwhich said ribbed faces and flat faces are bonded together by a materialhaving a lower refractive index than said glass sheets.

12. A block structure of light-conducting fibres comprising plural lightguiding fibres bonded together into a gas-tight slab having first andsecond surface on opposite sides thereof, the axes of said fibresextending along transverse lines from said first surface to said secondsurface, said fibres including ridged glass sheets having a flat faceand ridged face having ridges extending therefrom, said sheets arrangedto build up said fibres in the form of said block structure such thatpairs of adjacent ridged faces are superimposed with the respectiveridges interlocked and pairs of adjacent flat faces are disposed inabutting relation to each other, said ridged faces and flat faces beingbonded together by a material having a lower refractive index than saidsheets of glass.

13. In an electronic image-forming tube having an envelope, a fabricstructure forming at least part of said envelope and comprising plurallight guiding fibres joined together into a gas-tight slab, the axes ofsaid fibres extending along transverse lines from an interior surface ofsaid slab to an exterior surface thereof, said slab including wovenfabric sheets of said light guiding fibres and transverse connectivestrands, said woven fabric sheets being disposed on top of one anotherwith said fibres in parallel alignment to build up said fabric structureof said light guiding fibres, said fabric structure being bonded to saidenvelope in gas-tight fashion so as to form a structural wall of thetube.

'14. An electronic image-forming tube according to claim 13 in whichsaid transverse connective strands are of a lesser diameter than saidfibres.

15. An electronic image-forming tube according to claim 13 in which saidsheets are woven with said transverse strands forming the warp and saidlight guiding fibres forming the weft.

16. An electronic image-forming tube according to claim 15 in which saidfibres are tapered with each of said fibres having a substantially thinend region and thick end region, said warp strands interconnected atsaid thin end regions such that said thick end regions make contact witheach other.

17. An electronic image-forming tube according to claim 15 in which saidWarp strand passes alternately over two and under two weft strands withthe crossing point advancing by one weft strand to form a twill weave.

18. In an electronic image-forming tube having an envelope, a fabricstructure forming at least part of said envelope and comprising lightguiding fibres bonded together into a gas-tight slab, the axes of saidfibres extending along transverse lines from an interior surface of saidslab to an exterior surface thereof, said slab including woven fabricsheets of said light guiding fibres and transverse connective strands,said light-guiding fibres and transverse connective strands interwovenover and under said fibres to form fabric sheets of woven material, saidfabric sheets being disposed on top of one another with said fibres inparallel alignment to build up said fibres of said fabric structurewhich is bonded to said envelope in gas-tight fashion to form astructural wall of the tube.

19. In an electronic image-forming tube having an envelope, a fabricstructure forming at least part of said envelope and comprising plurallight guiding fibres joined together into a gas-tight slab, said lightguiding fibres coated with a material having a lower refractive indexthan said light guiding fibres, the axes of said fibres extending alongtransverse lines from an interior surface of said slab to an exteriorsurface thereof, said slab including woven fabric sheets of said lightguiding fibres and transverse connective strands, said woven fabricsheets being disposed on top of one another with said fibres in parallelalignment to build up said fabric structure of said light guidingfibres, said fabric structure bonded to said envelope in gas-tightfashion so as to form a structural wall of the tube.

20. A fabric structure of light conducting fibres for use in a tubehaving an envelope comprising plural light guiding fibres bondedtogether into a gas-tight slab having first and second surfaces onopposite sides thereof, the axes of said fibres extending alongtransverse lines from said first surface to said second surface, saidslab including woven fabric sheets of said light guiding fibres andtransverse connective strands, said woven fabric sheets being disposedon top of one another with said fibres in parallel alignment to build upsaid fabric structure of said light guiding fibres, said fabricstructure being bonded to said envelope in gas-tight fashion so as toform a structural wall of said tube.

21. A fabric structure of light conducting fibres according to claim inwhich said fibres are coated with a material having a lower refractiveindex than said fibres.

22. In an electronic image-forming tube having an envelope, a blockstructure of light-conducting fibres forming at least part of saidenvelope and comprising plural light guiding fibres bonded together intoa gastight slab, the axes of said fibres extending along transverselines from an interior surface of said slab to an exterior surfacethereof, said fibres including ridged glass sheets having a fiat faceand a ridged face having ridges extending therefrom, said sheetsarranged to build up said fibres in the form of said block structuresuch that said ridged faces or adjacent pairs of sheets are superimposed with the respective ridges interlocked and said flat faces ofadjacent pairs of sheets in abutting relation to each other, said blockstructure being bonded to said envelope in gas tight fashion so as toform a structural wall of said tube.

23. In an electronic image-forming tube having an envelope, at blockstructure of light-conducting fibres forming at least part of saidenvelope and comprising plural light guiding fibres bonded together intoa gastight slab, the axes of said fibres extending from an interiorsurface of said slab to an exterior surface there of, said fibresincluding ridged glass sheets having a fiat face and ridged face havingridges extending therefrom, said sheets arranged to build up said fibresin the form of said block structure such that pairs of adjacent ridgedfaces are superimposed with the respective ridges interlocked and pairsof adjacent fiat faces are disposed in abutting relation to each other,said ridged faces and fiat faces bonded together by a material having alower refractive index than said sheets of glass, said block structurebeing bonded to said envelope in gas tight fashion so as to form astructural wall of said tube.

24. In an electronic image forming tube having an envelope, a blockstructure of light-conducting fibres comprising plural light guidingfibres bonded together into a gas-tight slab including a thin membraneforming at least part of said envelope, said block structure havingfirst and second surfaces on opposite sides thereof, the axes of saidfibres extending along transverse lines from said first surface to saidsecond surface, said fibres including ridged sheets having a fiat faceand ridged face having ridges extending therefrom, said sheets arrangedto build up said fibres in the form of said block structure such thatthe ridged faces of adjacent pairs of sheets are superimposed with therespective ridges interlocked and the fiat faces of adjacent pairs ofsheets in abutting relation to each other, said block structure beingbonded to said membrane which forms a structural wall of said tube.

25. In an electronic image-forming tube having an envelope, at blockstructure of light-conducting fibres comprising plural light guidingfibres bonded together in to a gas-tight slab including a thin glassmembrane forming at least part of said envelope, said block structurehaving first and second surfaces on opposite sides thereof, the axes ofsaid fibres extending along transverse lines from said first surface tosaid second surface, said fibres including ridged sheets having a fiatface and a ridged face having ridges extending therefrom, said sheetsarranged to build up said fibres in the form of said block structuresuch that pairs of adjacent ridged faces are superimposed with therespective ridges interlock and pairs of adjacent fiat faces aredisposed in abutting relation to each other, said ridged faces and flatfaces bonded together by a material having a lower refractive index thansaid ridged sheets, said block structure being bonded to said membranewhich formed a structural wall of said tube.

26. An electronic image-forming tube according to claim 25 in which saidridged sheets and bonding material are glass.

27. An electronic image-forming tube according to claim 25 in which saidridged sheets are acrylic resin and said bonding material is epoxyresin.

28. In an image-forming device a woven light-guiding structure havingopposite faces and including plural woven fabric sheets extendingbetween said opposite faces, each of said woven fabric sheets includinglightguiding fibres forming the weft thereof and plural transversestrands forming the warp thereof, said woven fabric sheets beingdisposed one on top of another with said light-guiding fibres insubstantial alignment to build up a block of said light-guiding fibres.

References Cited by the Examiner UNITED STATES PATENTS 2,198,115 4/1940John 8824 2,354,591 7/1944 Goldsmith 88-1 X 2,825,260 3/1958 OBrien 88l3,041,228 6/1962 MacLeod 15667 3,141,105 7/1964 Courtney-Pratt 313-92 XFOREIGN PATENTS 179,905 10/1954 Austria.

JAMES W. LAWRENCE, Primary Examiner.

DAVID J. GALVIN, Examiner.

P. C. DEMEO, Assistant Examiner.

1. IN AN ELECTRONIC IMAGE-FORMING TUBE HAVING AN ENVELOPE, A HONEYCOMBSTRUCTURE FORMING AT LEAST PART OF SAID ENVELOPE AND COMPRISING LIGHTGUIDING FIBRES BONDED TOGETHER INTO A GAS-TIGHT SLAB, THE AXES OF SAIDFIBRES EXTENDING ALONG TRANSVERSE LINES FROM AN INTERIOR SURFACE OF SAIDSLAB TO AN EXTERIOR SURFACE THEREOF, SAID FIBRES INCLUDING CORRUGATEDGLASS SHEETS HAVING THE CORRUGATIONS THEREOF EXTENDING PARALLEL TO SAIDTRANSVERSE LINES, FLAT GLASS SHEETS INTERLEAVED BETWEEN SAID CORRUGATEDGLASS SHEETS AND COOPERATING TO FORM INTERNAL VOLUMES BETWEEN ADJA-